1. Field of the Invention
The present invention relates to an autofocus apparatus to be used for an optical apparatus such as a television camera lens or a video lens, and more particularly, to an autofocus apparatus to be used for an optical apparatus having an autofocus function and including a phase difference detecting type focus detector.
2. Description of the Related Art
Hitherto, various proposals have been made for autofocus (AF) technology for an image pickup apparatus such as a camera or a video camera. For instance, there is well known an autofocus method in which light beams from an object after passing through different exit pupil areas of an image pickup lens are focused to form an image on a pair of line sensors, and a defocus amount is calculated to drive the image pickup lens.
Japanese Patent Application Laid-Open No. 2001-021794 discloses a method of predicting a future image plane position by using a given function based on one of a focal length of an image pickup lens, an object distance, an object speed, an object acceleration, an image plane distance, an image plane speed, and an image plane acceleration, or a combination thereof.
In moving image photography using a camera or a video camera, it is necessary to adjust focus so that an object image is always within a depth of field. However, the conventional technology disclosed in Japanese Patent Application Laid-Open No. 2001-021794 does not consider individual variation of lens driving performance or a photographing condition which affects lens driving performance because the image pickup lens is driven based on the given function. In addition, because it is a precondition to take still images, it is not adaptive to taking moving images using a camera or a video camera.
FIG. 9 shows a locus of a focus position of a lens with respect to an object moving in an optical axis direction. In FIG. 9, the vertical axis represents an object distance, and the horizontal axis represents time. In FIG. 9, at points of time t1 and t2 as exposure points, an in-focus state is realized. However, in a period between the points of time t1 and t2 during which the exposure point is not reached, the in-focus state in which the focus position of the lens corresponds to the object position within the depth of field cannot be maintained.